Video display devices are nowadays widely used in articles such as televisions, computers, video games and the like. Many of them generally employ a cathode ray tube (CRT) which is a vacuum tube display device in which the image is created by electrons from an electron gun striking a phosphor screen that converts the electron energy into light energy over a wide wavelength range, usually the visible range for common display devices such as television and computer monitors. The CRT may be monochromatic (single color) or a color display device which produces images in more than one color, typically the three primary colors: red, green and blue.
A common problem with video display devices is the light reflected from the device towards the viewer, which generally fatigues the viewer's eyes. The reflected light consists of ambient light reflecting off the surface of the screen (which is typically a glass surface) as well as ambient light reflecting off the phosphors behind the screen. Several attempts have been made in the past to avoid or reduce this reflected light. U.S. Pat. No. 4,989,953, in column 2, line 13 through column 3, line 22, describes some of these earlier attempts and the problems associated with them. Most of these attempts, however, have succeeded in reducing the glare from monochromatic display monitors only.
For color displays, earlier attempts to reduce light reflection included, for example, use of a neutral density filter. Neutral density filters or attenuators are designed to produce attenuation that is uniform regardless of the wavelength. See, for example, Jeff Hecht, "The Laser Guidebook," 2nd edition, McGraw-Hill, Inc., New York, 1992, page 79. Such filters comprise colloidal suspensions of silver or graphite particles in a suitable medium and adhere to the monitor surface. This type of filter transmits a fraction of the light passing through it, independent of the wavelengths. In fact, neutral density filters are widely used in the manufacturing of current color CRT displays for lack of no better alternative. These filters, however, have the disadvantage of reducing the brightness of the image.
Another approach has been to use selective filtration by using different colored plates to absorb certain wavelengths. They, however, suffer the disadvantage that one has to use a different color filter for each phosphor element. Combining several filter materials in order to transmit just the desired red, green and blue generally results in the absorption of some of the desired wavelengths due to cascading of the different filter materials. This reduces the amount of red, green and blue that eventually gets transmitted.
Yet another approach involves a combination of a neutral density filter and an antireflection coating. While this cuts down the reflected light, it also reduces the brightness and the resolution of the image.
U.S. Pat. No. 5,121,030 discloses absorption filters which contain a transparent substrate with a plurality of spatially separated areas that contain selective absorptive dye colorants. Since this requires spaced areas with different dye components therein, the construction of the filter is quite complex and difficult to manufacture in large quantities.
U.S. Pat. No. 4,989,953 referred to above advocates the use of colored filters for monochromatic displays. Thus, for example, a magenta colored filter is used for CRTs with green phosphors, and a blue colored filter is used for amber colored CRTs. However, this concept is not much useful for color displays because the blue filter, for example, will block out the red and/or green depending on the spectral characteristics of the filter. The same problem exists for the other color filters that U.S. Pat. No. 4,989,953 discloses. If such filters are used for full color displays, the resulting display color will be severely distorted. For this reason, U.S. Pat. No. 4,989,953 suggests that a neutral density or gray colored filter must be used for multi-color or black and white displays. However, this approach, as stated before, reduces the brightness of the display. Since neutral density filters absorb a substantial amount of the desired light, the displays using neutral density filters must be capable of producing intense light. This was one of the reasons for developing super bright phosphors for display applications. Such bright phosphors substantially increase the cost of the display, however.
Another kind of visual display device being increasingly used is characterized as a plasma display panel (PDP). The basic mechanism of monochrome display operation is relatively simple. Inert gases, such as helium, neon, argon, xenon or mixtures thereof are hermetically sealed in a glass envelope and are subjected to a high voltage which causes the gas to ionize, producing a plasma. Color operation can also be achieved in a plasma display. Such operation utilizes ultraviolet light generated by the plasma discharge, rather than the glow of color of the plasma directly. Thus in color operation, phosphors are placed in the vicinity of the plasma discharge. The plasma-generated UV light hits the phosphors and generates visible light for the display. Plasma display panels, also known as gas display panels, have features such as a wide viewing angle, easy to see display because of self light emission, and a slim form. These advantages have encouraged increasing use of gas discharge display panels for high quality television sets. The exact structure of the PDPs is not a feature of the present invention, and it is contemplated that the filters of this invention are useful for any color PDP regardless of the exact configuration. Those of ordinary skill in the art would be capable of using the inventive filter with any PDP device.
Unfortunately, plasma displays currently being developed by various display manufacturers, still do not have high enough brightness nor high enough red, green and blue color transmission. Therefore, neutral density filters cannot effectively be used for color and contrast enhancement in plasma display applications since such filters would further reduce the brightness of the display. Additionally, since the sub-pixels of the phosphors are in close proximity to each other, there is a need for a physical barrier to prevent stimulation of a non-selected phosphor region. To achieve truer color emissions from the displays, devices such as very expensive circular polarizer-based contrast enhancing filters are being used.
Thus, in view of the varied uses and potential uses for CRTs and plasma display panels there is a need in the industry to have some device or mechanism to efficiently reduce the reflected light from the display devices as well as increase overall color and improve contrast and color enhancement without significantly sacrificing the brightness and resolution of the image.
It is, therefore, an object of this invention to provide a filter for color displays to reduce light reflected off such displays.
It is an additional object of this invention to provide a filter containing specific dye sets to enhance the contrast and color of images from a color display monitor without significantly sacrificing brightness of the image therefrom.
It is a further object of this invention to provide a spectrally tuned multiple bandpass filter for color displays, specifically matched to the three primary colors, namely red, green, and blue.
Other objects and advantages of this invention will be apparent to those skilled in the art from the accompanying description and examples.